Aromatic hydrocarbon recovery by extractive distillation, extraction and plural distillations

ABSTRACT

AROMATIC HYDROCARBONS ARE SEPARATED FROM A MIXTURE OF AROMATICS AND NON-AROMATICS, SUCH AS A C6-C8 ANPHTHA FRACTION, BY A COMBINATION OF PRELIMINARY FRACTIONATION, EXTRACTIVE DISTILLATION OF THE FRACTIONATION OVERHEAD, AND SOLVENT EXGTRACTION OF THE FRACTIONATION BOTTOMS.   D R A W I N G

H. L. THOMPSON AROMATIC HYDROCARBON RECOVERY BY EXTRACTIVE DISTILLATION,

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INVENTOR Herbert Lyf/e Thompson United States Patent Oifice U.S. Cl. 203-43 12 Claims ABSTRACT OF THE DISCLOSURE Aromatic hydrocarbons are separated from a mixture of aromatics and non-aromatics, such as a C -C naphtha fraction, by a combination of preliminary fractionation, extractive distillation of the fractionation overhead, and solvent extraction of the fractionation bottoms.

BACKGROUND OF THE INVENTION The present invention pertains to the extraction and recovery of aromatic hydrocarbons from a mixture of aromatic and non-aromatic hydrocarbons by the utilization of a sulfolane-type solvent. More particularly, the present invention pertains to the recovery and extraction of aromatic hydrocarbons from a naphtha fraction by a combination of feed fractionation, solvent extraction and extractive distillation wherein a sulfolane type chemical is utilized.

Conventional process for the recovery of high purity aromatic hydrocarbons such as benzene, toluene and xylenes (BTX) from various hydrocarbon feedstocks such as catalytic reformate, pyrolysis gasoline, etc., typically utilize an aromatic selective solvent such as sulfolane or diethyleneglycol. In these prior art processes, a hydrocarbon feedstock is first contacted in an extraction zone witth a solvent composition which selectively dissolves the aromatic components of the feedstock, thereby forming a rafiinate phase comprising one or more nonaromatic hydrocarbons and an extract phase comprising a solvent having aromatic hydrocarbons dissolved therein. The aromatic hydrocarbons contained in the extract may be recovered by distillation to yield an overhead distillate containing a portion of the extracted aromatics, an aromatic side out fraction and a solvent bottoms fraction suitable for reuse in the extraction zone. Also, not infrequently, the extract phase is subjected to extractive distillation to remove contaminating quantities of non-aromatics from the aromatics in order to make possible the recovery of nitration grade aromatic hydrocarbons. Further, added solvent is often added to the extractive distillation column in order to enhance the separation therein of the nonaromatic hydrocarbon contaminants from the desired aromatic hydrocarbons. The source of this added solvent is either fresh solvent introduced from the outside, a returning portion of the lean solvent from the solvent regeneration facilities or from a solvent recovery column associated with the total process. Accordingly, it is seen that in the typical prior are processes containing a solvent extraction step and an extractive distillation step, all of the aromatics present in the original feedstock are passed through the solvent extraction zone and the extractive distillation zone and that the latter zone utilizes a relatively pure solvent addition.

Further, as previously indicated, the two most prevalent methods of effecting a selective aromatic extraction utilize either a glycol or sulfolane type solvent. However, in the last decade, the industry has utilized a sulfolane type system as its preferred mode of operation because of certain well known desirable advantages which 3,723,256 Patented Mar. 27, 1973 a sulfolane system possesses in comparision to a glycol type system. More particularly, the industry has often found it to be advantageous to convert existing glycol type units into sulfolane type units. Typical of the prior art, glycol units which are being converted to a sulfolane type operation, is that process embodied within US. Pat. No. 2,730,558. This cited operation utilizes a solvent extraction zone and a stripping column which comprises a flash zone, a vaporizing zone and a stripping zone operating in a manner suflicient to substantially remove contaminating quantities of non-aromatic hydrocarbons from the extract phase. However, it has been difficult, in commercially operating plants, to fully utilize this type of stripping column when the operation is changed from a glycol type operation to a sulfolane type operation without the addition of other equipment.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a method for readily converting and operating existing prior art processes, originally built for glycol type solvents when converted to a sulfolane type of solvent.

It is a further object of this invention to provide a process for the recovery of aromatic hydrocarbons from a mixture of aromatic and non-aromatic hydrocarbons by a facile and economical combination of fractionation, selective solvent extraction and extractive distillation irrespective of the solvent type employed.

The objects of the present invention are accomplished by passing an aromatic containing hydrocarbon feed to a distillation column. Recovered overhead is a benzene containing, light fraction which is passed to an extractive distillation tower with the heavier bottoms fraction passed to a liquid extraction unit. The extract from the liquid extraction unit is stripped, in a stripping zone, of nonaromatic hydrocarbons to produce a non-aromatics free fraction and a non-aromatics containing fraction. The aromatics recovered in admixture with solvent from the extractive distillation tower are passed to a recovery section in admixture with the aromatic containing fraction from the stripping zone. The overhead stream from the extractive distillation column and the non-aromatics from the stripping zone are passed to the bottom section of the solvent extraction zoneto function as a reflux stream within this extraction zone. The recovery column produces a non-aromatic free aromatic product and produces a solvent suitable for used in both the solvent extraction unit and the extractive distillation tower.

In an embodiment, therefore, the present invention relates to the recovery of aromatic hydrocarbons from a hydrocarbon mixture containing non-aromatic and aromatic hydrocarbons including benzene and C aromatics by first introducing this hydrocarbon mixture into a fractionation zone maintained under fractional distillation conditions to provide a benzene stream containing nonaromatic hydrocarbons and a C aromatic stream containing non-aromatic hydrocarbons. Preferably, the benzene stream is relatively free of aromatic hydrocarbons having 7 or more carbon atoms and the C aromatic stream is relatively free of benzene. The benzene containing stream is then passed into an extractive distillation zone maintained under aromatic hydrocarbon extractive distillation conditions including the presence of a solvent capable of selectively dissolving the aromatic components contained in the benzene stream to provide a first, relatively non-aromatic free stream containing benzene and solvent and a first non-aromatic stream. The C aromatic stream is passed into a solvent extraction zone maintained under aromatic hydrocarbon extraction conditions including the presence of a solvent of the same type utilized in the extractive distillation zone and a hereinafter characterized reflux stream to provide an extract phase comprising solvent having aromatic hydrocarbons and a minor amount of non-aromatic hydrocarbons dissolved therein, and a raffinate phase comprising non-aromatic hydrocarbons. A preferred solvent for utilization in the solvent extraction zone and extractive distillation zone is sulfolane type chemical and in particular the chemical sulfolane. The extract phase from the solvent extraction zone is passed into a non-aromatic stripping zone maintained under stripping conditions to provide a second non-aromatic stream and a second, relatively non-aromatic free stream comprising solvent and aromatic hydrocarbons. The first non-aromatic stream from the extractive distillation zone and the second, non-aromatic stream from the non-aromatic stripping zone are passed into the solvent extraction zone as the specified reflux stream. The first, non-aromatic free stream from the extractive distillation zone and the second, non-aromatic free stream from the stripping zone are passed to a recovery separation zone maintained under separation conditions to provide a relatively non-aromatic free and solvent free extract stream and two solvent streams. One of these solvent streams is passed to the extractive distillation zone and the other solvent stream is passed to the solvent extraction zone. Preferably, this recovery separation zone is a fractionation zone maintained under fractionation conditions. While it is within the scope of this invention to recover the solvent streams as aliquot portions of a solvent bottoms fraction, a preferred mode of operation is to recover a solvent side cut stream containing a minor amount of aromatic hydrocarbons and passing this stream to the extractive distillation zone while recovering a relatively pure solvent stream as bottoms and passing this stream to the solvent extraction zone. The process of this invention is particularly suited to a naphtha fraction and particularly to C -C naphtha fractions containing greater than 75% by weight aromatic hydrocarbons.

In a more limited preferred embodiment, the present invention provides a process for the recovery of aromatic hydrocarbons from a hydrocarbon mixture containing non-aromatic hydrocarbons and aromatic hydrocarbons including benzene and C aromatics by first fractionating this hydrocarbon mixture to provide a benzene containing stream, also containing non-aromatic hydrocarbons and a C aromatic containing stream, also containing non-aromatic hydrocarbons. The benzene containing stream is passed into the extractive distillation zone maintained under extractive distillation conditions includng the presence of sulfolane solvent to provide a first, relatively non-aromatic free stream containing benzene and sulfolane solvent and a first non-aromatic stream. The C aromatic stream is passed into a solvent extraction zone also maintained under extraction conditions including the presence of sulfolane solvent, and a hereinafter characterized reflux stream, to provide an extract phase comprising sulfolane solvent having aromatic hydrocarbons and non-aromatic hydrocarbons dissolved therein and a raifinate phase comprising non-aromatic hydrocarbons. The extract phase from the solvent extraction zone is passed into a non-aromatic stripping zone maintained under stripping conditions to provide a second, non-aromatic stream comprising non-aromatic hydrocarbons and a second, relatively non-aromatic free stream comprising sulfolane solvent and aromatic hydrocarbons. The first non-aromatic stream from the extractive distillation zone and the second, non-aromatic stream from the stripping zone are then passed to the solvent extraction zone as the hereinbefore specified reflux stream. The first, non-aromatic free stream from the extractive distillation zone and the second, non-aromatic free stream from the stripping zone are passed to a recovery fractionation zone to provide a relatively non-aromatic free and sulfolane solvent free extract stream, a sulfolane solvent side cut stream containing a minor amount of aromatic hydrocarbons and a relatively pure sulfolane solvent stream. The side cut stream so produced is passed to the extractive distillation zone and the relatively pure solvent stream is passed to the solvent extraction zone.

Other objects, embodiments and a more detailed description of the foregoing embodiments will be found in the following more detailed description of the present invention.

DETAILED DESCRIPTION OF THE INVENTION A hydrocarbon feedstock containing non-aromatic hydrocarbons and aromatic hydrocarbons including benzene and C aromatics suitable for separation according to the process of this invention, include fluid mixtures having sulficiently high concentrations of aromatic hydrocarbons to economically justify the recovery of the aromatic hydrocarbons contained therein as a separate product stream. While the present invention is applicable to hydrocarbon feed mixtures which contain at least 25% by weight aromatic hydrocarbons, a particularly preferred feed mixture contains at least 75% by weight aromatics. A suitable carbon number range for the hydrocarbon feed mixture is from about 6 carbon atoms per molecule to about 20 carbon atoms per molecule, and more preferably from about 6 to about 10 carbon atoms per molecule. A suitable hydrocarbon feed source is the debutanized reactor effluent from a conventional catalytic reforming unit. Another suitable feedstock source is the liquid byproduct from a pyrolysis gasoline unit which has been hydrotreated to saturate olefins and diolefins thereby producing an aromatic hydrocarbon concentrate suitable for the solvent extraction techniques hereinafter described. Typically, the feedstock from a catalytic reforming process or from pyrolysis gasoline units contain single ring aromatic hydrocarbons comprising a wide boiling mixture of benzene, toluene and xylenes. These single ring aromatic hydrocarbons are also mixed with parafiins and naphthenes in the corresponding boiling range. A particularly preferred feedstock is naphtha fraction and, in particular, a hydrotreated liquid by-product from a pyrolysis gasoline unit or a debutanized efiiuent from a catalytic reforming unit containing at least about 75% aromatic hydrocarbons by weight. Particularly preferred is a C -C naphtha fraction.

Solvents capable of selectively dissolving and extracting aromatic hydrocarbons from a mixture of aromatic and non-aromatic hydrocarbons are well known to those trained in the art. A preferred solvent is a solvent of the sulfolane type. The sulfolane type solvent possesses a 5 member ring containing one atom of sulfur and 4 atoms of carbon with 2 oxygen atoms bonded to the sulfur atom of the ring. Generically, the sulfolane type solvents have a structural (formula as noted by Formula 1:

wherein R R R and R are independently selected from the group comprising a hydrogen atom and alkyl group having from 1 to 10 carbon atoms, an alkoxy radical having from 1 to 8 carbon atoms, and an arylalkyl radical having from 1 to 12 carbon atoms.

Other solvents analogous to the sulfolane type solvents which may be included within this process are sulfolenes such as 2-sulfolene or 3-sulfo1enes which have the structure as illustrated in formulas 2 and 3:

Other typical solvents which have a high selectivity for separating aromatics from non-aromatic hydrocarbons and which may be processed within the scope of the present invention are Z-methylsulfolane, 2,4-dimethylsulrfolane, methyl 2-sulfonyl ether, n-aryl-3-sulfonyl amine, 2- sulfonyl acetate, diethylene glycol,- various polyethylene glycols, dipropylene glycol, various polypropylene glycols, dimethyl sulfoxide, N-methyl pyrollidone, etc.

A specifically preferred solvent chemical to be utilized in the present invention is the chemical sulfolane wherein according to formula 1, R R R and R each comprise a hydrogen atom. The structural formula of sulfolane is set out in Formula 4:

Since these aromatic selective solvents are well known to those trained in the art and, in particular, since sulfolane type solvents are articles of commerce widely utilized in the solvent extraction art, greater detail thereon need not be presented herein.

The aromatic selectivity of aromatic selective solvents such as sulfolane can usually be enhanced by the addition of water to the solvent. Preferably the solvent utilized in the practice of this invention contains small quantities of water to increase the selectivity of the overall solvent phase for the aromatic hydrocarbons without reducing in a substantial manner, the solubility of the solvent in the aromatics. Further, the presence of water in the solvent composition provides a relatively volatile material which can be distilled from the solvent to vaporize the last traces of non-aromatic hydrocarbons from the solvent stream by steam distillation. Accordingly, a preferred solvent composition to be utilized in the process of the present invention contains about 0.1% to about 20% by weight water and more particularly from about 0.5 to about 1% by weight, depending upon the particular solvent utilized and the process conditions at which the solvent extraction zone and extractive distillation zone are operated.

According to the process of the present invention, the hydrocarbon feedstock is first fractionated, by means well known to those trained in the art, to provide an overhead fraction stream comprising benzene and non-aromatic hydrocarbons boiling within the benzene boiling range. Preferably, this fraction is relatively free of C and heavier aromatics and, in particular, it is preferred that this fraction contain atleast 95% mole benzene, on an aromatic basis. Also produced is a bottoms fraction stream containing C aromatics and non-aromatics within the xylene boiling range. Preferably, the bottoms fraction is relatively free of benzene and, in particular, it is preferred that this fraction contain less than 5 mole percent benzene, on an aromatic basis.

The aromatic content in the fractionation column overhead is recovered by passing the overhead fraction to an extractive distillation column maintained under extractive distillation conditions including the presence of a solvent of the type previously discussed, capable of selectively dissolving the aromatic components contained in the overhead fraction. This extractive distillation zone is not, per se, a solvent extraction, since the solvent serves to alter the relative volatility between the aromatic and non-aromatic hydrocarbons so that a separation by fractional distillation may be elfected.

Extractive distillation conditions and techniques are generally well known to those trained in the art, and vary, depending on the particular aromatic selective solvent utilized. Therefore, for brevity, from here on the discussion will be generally limited to sulfolane solvent and no intent is made thereby to unduly limit the scope of this invention to this singular solvent species.

Broadly speaking, extractive distillation conditions for extracting the benzene and any other aromatics present in the overhead from the non-aromatic hydrocarbons (parafiins and naphthenes) include moderate pressures and sufficiently high temperatures in order to vaporize all of the non-aromatic hydrocarbons and to provide a predominantly non-aromatic fraction stream containing non-aromatic hydrocarbons and, typically, a minor amount of aromatic hydrocarbons, usually benzene. Provided also is a relatively non-aromatic free bottoms fraction stream containing aromatic hydrocarbons and solvent. Typically, this bottoms fraction contains less than 1000 p.p.m. and preferably less than 500 p.p.m. by weight non-aromatics. Typical operating conditions in the extractive distillation zone when utilizing a sulfolane solvent, include a pressure of about mm. Hg to 40 p.s.ig., an overhead temperature of about F. to about 330 F. and a bottoms temperature of about F. to about 355 F. Sulfolane solvent to feed ratios can vary from about 1:1 to about 20:1, depending, as do the pressure and temperature, on feed composition. Preferred solvent to feed ratios when processing a C -C naphtha cut are about 2:1 to about 6:1.

The C aromatic bottoms fraction stream is passed into a solvent extraction zone, maintained under aromatic hydrocarbon extraction conditions including the presence of a solvent of the same type utilized in the extractive distillation zone which, in the preferred embodiment, is sulfolane. Provided is an extract phase comprising solvent having aromatic hydrocarbons and a minor amount of non-aromatic hydrocarbons dissolved therein, and a raflinate phase comprising non-aromatic hydrocarbons. Typically, the raffinate is water washed to remove any sulfolane which may be entrained therein. Preferably, the extraction conditions utilized are correlated to maintain the solvent and hydrocarbons passed to the extraction zone in the liquid phase so as to embody a liquid phase solvent extraction. As in the case of the extractive distillation zone, the conditions, apparatus and mode of operation associated with the solvent extraction zone are well known to those trained in the art. For example, see Petroleum Refiner, No. 8, vol. '38, September 1959, pages 185-192, the teachings of which are specifically incorporated by reference herein.

Also embodied with in the solvent extraction zone is the concept of displacing non-aromatic hydrocarbons from the extract phase at the lower end of the solvent extraction zone by utilizing the known technique of a non-aromatic hydrocarbon containing reflux at that point. It is preferred that this reflux stream comprise relatively light non-aromatic hydrocarbons but significant quantities of aromatic hydrocarbons, i.e., 30 to 60% by weight may be present. The exact amount of reflux introduced into the lower section of the solvent extraction zone varies, depending on the degree of non-aromatic hydrocarbon rejection desired in the extraction zone. Preferably, the reflux is at least 10% by volume of the extract phase so as to insure effective displacement of the non-aromatic hydrocarbons from the extract phase into the raflinate. According to the process of the present invention, at least a portion, if not all, of the reflux required in the solvent extraction zone is provided by the nonaromatic overhead stream provided by the extractive distillation zone preferably after condensation and water removal and a non-aromatic containing stream provided by a hereinafter described non-aromatic stripping zone.

The solvent extraction zone is operated under conventional conditions including elevated temperatures and a sufiiciently elevated pressure to maintain the solvent, reflux streams and hydrocarbon charge in the liquid phase. When utilizing sulfolane, suitable temperatures are about 80 F. to about 400 F, preferably about F. to about 300 F, and suitable pressures are about atmospheric to about 400 p.s.i.g., preferably about 50 to 150 p.s.i.g. Solvent quantities should be suflicient to dissolve substantially all of the aromatic hydrocarbons present in the solvent extraction zone feed. Preferred are solvent to feed ratios, by volume, of about 2:1 to about :1 when utilizing a C -C range naphtha cut as feed.

The extract from the solvent extraction zone containing aromatic hydrocarbons, solvent and a lesser amount of non-aromatic hydrocarbons, i.e., less than 30-50 mole percent on a hydrocarbon basis, is passed into a nonaromatic stripping zone to remove therefrom essentially all of the non-aromatic hydrocarbons. The non-aromatic stripping zone is maintained under stripping conditions well known to those trained in the art, including moderate pressures and sufiiciently high reboiler temperatures in order to vaporize all of the non-aromatic hydrocarbons and, as a result, a portion of the solvent and aromatic hydrocarbons from the bottoms portion of the stripping zone. This is necessary to produce a bottoms fraction stream relatively free from non-aromatic hydrocarbons and containing aromatic hydrocarbons and solvent. The non-aromatics are removed overhead, in admixture with some solvent and aromatic hydrocarbons, and are passed, usually after condensation and water removal to the hereinbefore described solvent extraction zone to provide at least a portion of specified reflux stream.

While the particular stripping zone utilized is not critical to the practice of the present invention, the process of the present invention is particularly suited to the utilization of the stripping zone contained in the hereinbefore described prior art glycol type extraction, (U.S. Pat. No. 2,730,558). However, an extractive type stripper as utilized in conventional, grass roots, sulfolane units is equally suited to the process of the present invention. Such a stripping zone will be maintained under a pressure of atmospheric to about 100 p.s.i.g. although a top pressure of about 5 p.s.i.g. to about p.s.i.g. is preferred. The reboiler temperature is a function of feed composition and is preferably maintained at a level sufficient to produce a bottoms fraction containing less than 2000 p.p.m. weight nonaromatics on a hydrocarbon basis. Maximum stripping is accomplished by introducing the extract feed into the upper portion of the stripping zone in a manner well known to those trained in the art.

When utilizing a stripping zone as exemplified in U.S. Pat. No. 2,730,558, the solvent extraction zone extract is first passed into a flash zone wherein a portion of the non-aromatics is flashed off. The remaining extract is then passed into a vaporizing section wherein another portion of the non-aromatics are vaporized. Finally, the residue of the extract stream, now comprising sulfolane solvent and aromatics but still containing a minor amount of non-aromatics, is passed to a lowermost stripping section wherein a true stripping operation of the remaining nonaromatics takes place. The bottom of the stripping zone is typically maintained at a temperature of about 200 F. to 500 F., preferably a temperature of about 300 F. to about 425 F. The non-aromatic hydrocarbons recovered from the various sections of the stripping zone are condensed and usually after intermediate water removal, passed as reflux to the solvent extraction zone. If desired, additional sulfonlane solvent beyond that contained in the original extract may be added to the upper portion of the stripping section to enhance aromatic recoveries therein and for improving non-aromatic rejection by improving the selectivity.

The aromatics contained in the non-aromatic free stream provided by the non-aromatic stripping zone and the non-aromatic free stream provided by the extractive distillation zone, are recovered from the solvent by passing each fraction to a recovery separation zone maintained under separation conditions sufiicient to provide a relatively non-aromatic free and sulfolane solvent free aromatic extract stream and at least two solvent streams. Pref- 8 erably, the aromatics recovered contain less than 1000 p.p.m. by weight non-aromatic hydrocarbons.

Preferably, the recovery separation zone is a fractionation zone and may typically utilize steam as an input stripping medium to aid in the separation of sulfolane solvent from the aromatic hydrocarbons. When the separation zone is, in the preferred embodiment, a fractionation zone, this zone is maintained under fractionation conditions, generally well known to the art, such as low pressures and sulficiently high temperatures to distill the aromatic hydrocarbons overhead and to provide a relatively non-aromatic free and sulfolane solvent free, extract product stream. While, as in the case of the previously described zones utilized in the process of the present invention, the exact fractionation conditions depend on the solvent composition and particular hydrocarbon present, when processing a C -C naphtha cut, the top of the recovery fractionation zone is preferably maintained at a pressure of about to about 400 mm. Hg absolute since subatmospheric pressures must be employed to maintain a sufficiently low reboiler temperature to avoid thermal decomposition of the sulfolane solvent.

As indicated, the recovery fractionation zone also provides at least two, relatively pure solvent streams. These streams contain the solvent passed to the recovery zone and provide at least a portion of the solvent required in the extractive distillation zone and in the solvent extraction zone. The recovery fractionation zone can be operated to produce a single bottoms fraction stream relatively free of aromatic hydrocarbons which can be divided into aliquot portions of equal composition. However, a preferred mode of operation, when the recovery zone comprises a fractionation zone, is to withdraw a solvent side cut stream comprising solvent and some aromatic hydrocarbons in addition to the relatively pure solvent bottoms streams. Preferably, when processing a C -C naphtha cut, this side stream is withdrawn from a point intermediate to the feed point and the bottoms. The side cut stream is then passed to the extractive distillation zone wherein the aromatics contained therein are eventually recovered without aromatic loss. This mode of operation yields a particularly eflicient operation since the solvent extraction zone requires essentially pure solvent to eliminate and/or minimize the loss of C and heavier aromatics in the raflinate phase produced in this zone. However, since the aromatic content of the hydrocarbon feed to the extractive distillation zone is predomonantly benzene and is relatively free of C aromatics, high solvent purities of the degree required in the solvent extraction zone are not necessary. In addition, the minor amount of aromatics contained in this solvent side cut stream are not lost from the complete, overall recovery system since they will typically be recovered in the bottoms stream removed from the extractive distillation zone, and the minor amount of aromatics which may be present in the overhead stream from the extractive distillation are recovered when this overhead is used as reflux in the solvent extraction zone. Hence, since all of the solvent to be removed from the recovery column need not be essentially pure, a smaller recovery column is required and a more efficient economical process results.

The process of the present invention can be more fully explained and its benefits realized by reference to the following description of the appended drawing.

DESCRIPTION OF THE DRAWING The process of the present invention can be most clearly described and illustrated by reference to the attached schematic drawing illustrating the recovery of aromatic hydrocarbons from a hydrotreated C -C pyrolysis naphtha cut. Of necessity, certain limitations must be present in a schematic diagram of the type presented and no intention is made thereby to limit the scope of this invention as to feedstock materials, rates,

operating conditions, etc. Miscellaneous appurtenances including vessels, controls, pumps, compressors, separators, reboilers, reflux lines, etc., have been eliminated. Only those vessels and lines necessary for a complete and clear understanding of the embodiments contained in the present invention are included. Various modifications to the process variable and process flow made by those possessing expertise in the art of aromatic extraction are all included within the generally broad scope of the present invention.

A hydrotreated- C -C pyrolysis naphtha cut containing about 18% non-aromatic hydrocarbons is passed via line 1 to fractionation column 2 wherein the benzene and benzene boiling range non-aromatic hydrocarbons are separated therefrom. More specifically, fractionation column 2 maintains a bottoms temperature of about 315 F. and an overhead temperature of about 210 F. and a reflux ratio suflicient to produce an overhead product which, when removed via line 3, contains approximately 99% by weight benzene and less than 1% by weight toluene on an aromatic basis.

This fractionation column 2 overhead is passed via line 3 to an extractive distillation zone comprising extractive distillation column 4, condenser 20 and overhead accumulator 21. Within extractive distillation column 4, the majority of the aromatic hydrocarbons contained in line 3 are removed as bottoms via line 5 when subjected to extractive distillation conditions including the addition of sulfolane solvent containing some water via line 7 to the uppermost portion of column 4. More particularly, extractive distillation colunm 4 is maintained at a bottoms temperature and pressure of 350 F. and 37 p.s.i.g., the sulfolane solvent enters via line 7 from a source hereinafter to be described, and at a temperature of 245 F. and in amounts sufiicient to provide a 3.65:1 solvent to feed mole ratio and the overhead section is maintained at a temperature of about 270 F. and pressure of 27 p Removed as bottoms via line 5 from extractive distil lation column 4, is a sulfolane, aromatic mixture containing less than 200 ppm. weight non-aromatic hydrocarbons on a hydrocarbon basis. The overhead removed from extractive distillation column 4 is passed via line 6 through condensing means 20 and passed via line 6 to accumulator 21. Within accumulator 21, water entrained in the original sulfolane solvent is removed via line 22 and the resultant non-aromatic containing overhead is removed and passed via line 23 to the lower portion of solvent extraction zone 8 to serve as a light raflinate reflux therein.

The bottoms removed from fractionation zone 2 via line 9 are relatively free of benzene and comprise C and C aromatic hydrocarbons and the corresponding boiling range paraflins and naphthenes not removed as overhead from column 2. This bottoms stream is then passed via line 9 to solvent extraction zone 8 wherein the aromatic are recovered. More particularly, relatively pure'sulfolane solvent containing water is passed to the upper portion of solvent extraction zone 8 via line 19 and is contacted in a countercurrent fashion with the hydrocarbon feed entering via line 9. Specifially, proc essing conditions utilized in solvent extraction zone 8 include a temperature of 200 F. and pressure of 85 p.s.i.g. and a solvent to feed mole ratio of about 4.6 to 1. Removed from the upper portion of solvent extraction zone 8 is raflinate stream 10 which contains up to about 5% aromatic hydrocarbons and trace amounts of sulfolane solvent. This solvent contained in the non-aromatic raffinate stream 10 is recovered by water washing the raflinate by means not shown. To further help attain high purity, aromatic products, in addition to reflux stream 23, reflux stream 27 containing non-aromatic hydrocarbons in admixture with a minor amount of aromatics and sul- 10 fonate solvent, is introduced via line 27, the source of which will be described hereinafter.

Removed from the lower portion of solvent extraction zone 8 is aromatic extract stream ll which is passed to stripper zone 12. Stripper zone 12, as illustrated, is essentially a distillation zone containing two vaporization sections which function to flash off and vaporize a portion of the non-aromatc hydrocarbon contaminants contained in extract phase in line 11. Stripper zone 12 comprises an uppermost primary flashing section 12a, a lower or intermediate flashing or vaporizing section 12b, and a lowermost true stripping section wherein true stripping of the rich aromatic containing solvent is obtained. More particularly, flashing section 12a is maintained at a temperature of about 250 F. and a pressure of 5 p.s.i.g. and the lower stripping section 12c is maintained at a bottoms temperature of 350 F. and a pressure of 10 p.s.i.g.

As to the operation of stripper zone 12, rich aromatic extract enters flash zone 12a and, under the conditions maintained therein, a portion of the non-aromatic hydrocarbons is flashed oir" and removed via line 15. The remaining portion of the extract phase is removed via line 13 and passed to intermediate vaporizing section 12b wherein another portion of the non-aromatic hydrocarbons are vaporized and removed from the stripper zone as a vapor stream via line 14. Finally, the residue of the extract stream now comprising sulfolane solvent having the desired aromatic hydrocarbon dissolved therein, but still containing non-aromatic hydrocarbons, is passed into the lowermost stripping section 120 wherein the remaining non-aromatic hydrocarbons are removed therefrom via line 28 and admixed with those non-aromatic hydrocarbons contained in line 14. The non-aromatic hydrocarbons removed from stripper zone 12 via line 28, 14 and 15 are then passed via line 15 to condensing means 24 and passed via line 15 to accumulator 25. Removed from the bottom portion of accumulator 25 is water stream 26 which is passed to a solvent recovery means not illustrated to remove the trace amounts of solvent contained therein. The non-aromatic hydrocarbons are removed via line 27 and passed, as hereinbefore described, to the lower section of solvent extraction zone 8 to serve as a displacement reflux stream therein. If desired, added amounts of solvent may be introduced to the upper portion of stripping zone 120 to help insure an essentially non-aromatics free bottoms fraction.

Removed as bottoms from stripper zone 12 via line 16 is an aromatic hydrocarbon, sulfolane mixture which is combined with the aromatic hydrocarbon, sulfolane mixture removed as bottoms from extractive distillation column 4 via line 5. The resulting mixture is passed via line 5 to recovery column 17 for separation therein of the aromatic hydrocarbons and the sulfolane solvent; More particularly, recovery column 17 is operated at an overhead temperature and pressure of 164 F. and mm. Hg absolute, and a bottoms temperature and pressure of 350 F. and 440 mm. Hg absolute. Removed as a side cut stream from recovery column 17 is sulfolane stream 7 which is passed as the sulfolane source hereinbefore described for extractive distillation zone 4. More particularly, the feed to recovery column 17 enters at about plate 16 of a 30 plate fractionation column and side cut stream 7 is removed at about plate 25 of the column. Removed as bottoms from recovery column 17 is relatively pure solvent stream 19 which is passed, via line 19 as the hereinbefore described fresh sulfolane source passed to the upper portion of solvent extraction zone 8. If desired, additional, pure make up solvent may be added at this point and/or a portion of the solvent withdrawn and regenerated. By utilizing this combination of side cut stream and bottoms removal, an efficient separation with a minimum size recovery column is attained since the removal of the side cut stream increases the vapor to liquid ratio in the bottom portion of recovery column 17 so as to insure that a high purity sulfolane stream is passed to solvent extraction zone 8 and minimize the aromatic losses from the upper portion of this solvent extraction zone. The aromatic hydrocarbons are removed as a final aromatic hydrocarbon extract product via line 18 and are passed to fractionation means not shown, to recover the individual benzene, toluene and C aromatic hydrocarbon species. More particularly, the benzene contained in extract stream 18 represents 99.9% of the original benzene contained in feed stream 1 and is recoverable at a non-aromatic concentration of less than 200 p.p.m. Similarly, extract stream 18 contains a 98.9% of the original feed toluene at a non-aromatic level of less than 200 ppm, and the C aromatics represent 96.7% of the original feed aromatic at a nonaromatic level of less than 1000 p.p.m.

I claim as my invention:

1. A process for the recovery of aromatic hydrocarbons from a hydrocarbon mixture containing non-aromatic hydrocarbons and aromatic hydrocarbons including benzene and C aromatics which comprises the steps of:

(a) introducing said mixture into a fractionation zone maintained under fractionation conditions to provide a benzene stream containing non-aromatic hydrocarbons and a C aromatic stream containing non-aromatic hydrocarbons;

(b) passing said benzene stream into an extractive distillation zone maintained under aromatic hydrocarbon extractive distillation conditions including the presence of a solvent capable of selectively dissolving the aromatic component of said mixture to provide a first, relatively non-aromatic free stream containing benzene and solvent and a first non-aromatic stream;

(c) passing said C aromatic stream into a liquid phase solvent extraction zone maintained under aromatic hydrocarbon extraction conditions including the presence of a solvent of the type utilized in said extractive distillation zone and a hereinafter defined reflux stream to provide an extract phase comprising solvent having aromatic hydrocarbons and nonaromatic hydrocarbons dissolved therein and a raffinate phase comprising non-aromatic hydrocarbons;

(d) passing said extract phase from said solvent extraction zone into a non-aromatic stripping zone maintained under stripping conditions to provide a second non-aromatic stream comprising non-aromatic hydrocarbons and a second relatively non-aromatic free stream comprising solvent and aromatic hydrocarbons;

(e) passing said first non-aromatic stream from said extractive distillation zone and said second nonaromatic stream from said stripping zone into said solvent extraction zone as said specified reflux stream;

(f) passing said first non-aromatic free stream from said extractive distillation zone and said second, nonaromatic free stream from said stripping zone to a recovery separation zone maintained under separation conditions to provide a relatively non-aromatic free and solvent free extract stream and two solvent streams; and

(g) passing one of said solvent streams to said extractive distillation zone and passing one of said solvent streams to said solvent extraction zone.

2. The process of claim 1 wherein said solvent comprises a sulfolane type chemical of the general formula:

O (3% \CH--R4 R1CH--CHR:4

wherein R R R and R are independently selected from the group consisting of a hydrogen atom, an alkyl group having from 1 to 10 carbon atoms, an arylalkyl radical having from 1 to 12 carbon atoms, and an alkoxy radical having from 1 to 8 carbon atoms.

3. The process of claim 1 wherein said solvent comprises sulfolane. l

5 4. The process of claim 1 wherein said solvent comprises a sulfolene selected from the group consisting of 2-sulfolene and 3-sulfolene.

5. The process of claim 1 wherein said recovery zone comprises a fractionation zone maintained under fractionation conditions and said solvent streams are aliquot portions of a solvent bottoms fraction.

6. The process of claim 1 wherein said recovery zone comprises a fractionation zone maintained under fractionation conditions and said solvent stream passed to said extractive distillation zone is a solvent side cut stream containing a minor amount of aromatic hydrocarbons and said solvent stream passed to said solvent extraction zone, is a relatively pure solvent stream.

7. The process of claim 1 wherein said hydrocarbon mixture is a C C naphtha fraction.

8. A process for the recovery of aromatic hydrocarbons from a hydrocarbon mixture containing non-aromatic hydrocarbons and aromatic hydrocarbons including benzene and C aromatics which comprises the steps of:

(a) introducing said mixture into a fractionation zone maintained under fractionation conditions to provide a benzene containing stream, also containing nonaromatic hydrocarbons and a C aromatic containing stream also containing non-aromatic hydrocarbons;

(b) passing said benzene containing stream into an extractive distillation zone maintained under aromatic hydrocarbon extractive distillation conditions including the presence of a sulfolane solvent to provide a first, relatively non-aromatic free stream containing benzene and sulfolane solvent and a first nonaromatic stream;

(c) passing said C aromatic stream into a liquid phase solvent extraction zone maintained under aromatic hydrocarbon extraction conditions including the presence of sulfolane solvent and a hereinafter defined reflux stream to provide an extract phase comprising sulfolane solvent having aromatic hydrocarbons and non-aromatic hydrocarbons dissolved therein and a raifinate phase comprising non-aromatic hydrocarbons;

(d) passing said extract phase from said solvent extraction zone into a non-aromatic stripping zone maintained under stripping conditions to provide a second non-aromatic stream comprising non-aromatic hydrocarbons and a second, relatively non-aromatic free stream comprising sulfolane solvent and aromatic hydrocarbons;

(e) passing said first non-aromatic stream from said extractive distillation zone and said second nonaromatic stream from said stripping zone into said solvent extraction zone as said specified reflux stream;

(f) passing said first, non-aromatic free stream from said extractive distillation zone and said second, nonaromatic free stream from said stripping zone to a recovery fractionation zone maintained under fractionation conditions to provide a relatively non-aromatic free and sulfolane solvent free extract stream, a sulfolane solvent side cut stream containing a minor amount of aromatic hydrocarbons and a relatively pure sulfolane solvent stream; and

(g) passing said side cut stream to said extractive distillation zone and passing said relatively pure solvent stream to said solvent extraction zone.

9. The process of claim 8 wherein said hydrocarbon 7 mixture is a naphtha fraction.

10. The process of claim 8 wherein said benzene stream of step (a) is relatively free of aromatic hydrocarbons having 7 or more carbonatoms.

11. The process of claim 8 wherein said C aromatic 75 stream of step (a) is relatively free of benzene.

13 14 12. The process of claim 8 wherein said hydrocarbon 3,642,614 2/1972 Van Tassel] 260674 SE mixture is a C -C naphtha fraction. 3,537,984 11/1970 Thompson 260674 SE References Cited WILBUR L. BASCOMB, ]R., Primary Examiner UNITED STATES PATENTS 5 Us. Cl. X'R. 3,114,783 12/1963 Butler et al. 20358 2,775,627 12/1956 Lavender, Jr. 260674 SE 20348 82, 84; SE, 674 A 

